Custom image format importer
Normally you should implement the Operation as a wrapper on existing code. This way you can call an internal method what can return more information. This is also faster as operations creates undo files.

def load_dtx_texture(filepath):
        dtx = DTX(filepath)
        image_name = os.path.splitext(os.path.basename(filepath))[0]
        image = bpy.data.images.new(image_name, dtx.width, dtx.height, alpha=True)
        image.pixels[:] = dtx.pixels[:]
        return image

class MyTextureImporter(Operator):
    def execute(self, context):
        load_dtx_texture(self.filepath)
        return {'FINISHED'}

class MyMeshImporter(Operator):
    def execute(self, context):

        ....

        image = load_dtx_texture(self.filepath)

        ....

        return {'FINISHED'}